The present invention relates to an apparatus and a method for controlling a braking force provided to each wheel of a vehicle.
Brake control apparatuses have been known that execute brake control for reducing the turning radius when a vehicle such as a general automobile turns. The brake control apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-207823 (hereinafter, referred to as a conventional apparatus), for example, obtains the steering angle of the steering wheel and determines that the steering wheel is in the maximum steering state in the case where the thus obtained steering angle is equal to or greater than a preset threshold value. In this case, this conventional apparatus executes brake control during turning (this is also referred to as small turn control) in order to provide a braking force to the rear wheel on the inner side relative to the direction in which the vehicle turns (for example, the right rear wheel in the case where the vehicle is turning to the right). The maximum steering state means a state where the steering wheel is steered in one turning direction (clockwise or counterclockwise) to the maximum.
The vehicles which carry out the small turn control in this manner turn with a turning movement around the rear wheel on the inner side relative to the direction in which the vehicle turns. Therefore, the turning radius of the vehicles which carry out the small turn control becomes smaller than that of the vehicles which do not carry out the small turn control. As a result, when a driver finds an obstacle in the direction in which the vehicle is traveling, and turns the steering wheel to the maximum, it is possible to avoid the obstacle appropriately by carrying out the small turn control.
In vehicles in which a conventional apparatus as described above is mounted, in the case where the steering wheel is turned to the maximum even in a state where the vehicle is not moving, a braking force is provided to the rear wheel on the inner side relative to the direction in which the vehicle turns by carrying out the small turn control. In the case where the accelerator pedal is depressed by the driver of the vehicle in this state, the vehicle turns in a state where a braking force is provided to the rear wheel on the inner side relative to the direction in which the vehicle turns. The magnitude of the braking force provided to the rear wheel on the inner side relative to the direction in which the vehicle turns is set assuming a state where the wheels of the vehicle are being driven.
In general, however, the coefficient of static friction is greater than the coefficient of dynamic friction, and this also applies to the brake pads for vehicles. Therefore, in the case where a vehicle in a stopped state starts running through the depressing process of the accelerator pedal by the driver while the small turn control is being carried out, the type of friction force provided to the brake pads is switched from the static friction force to the dynamic friction force, and thus, the braking force applied to the rear wheel on the inner side relative to the direction in which the vehicle turns is rapidly reduced. That is to say, the rotational speed of the rear wheel on the inner side relative to the direction in which the vehicle turns is rapidly increased. As a result, an increase in the acceleration in the direction in which the vehicle is traveling delays relative to the amount by which the driver depresses the accelerator pedal (or the pedal operation speed) when the small turn control is carried out immediately after the vehicle starts moving. This may cause acceleration to feel sluggish. Sluggishness refers to when the acceleration experienced by the driver is less than that intended by the driver.
In addition, in the case where the vehicle starts moving in a state where a braking force is provided to the rear wheel on the inner side relative to the direction in which the vehicle turns, noise may be made in the vicinity of the rear wheel on the inner side relative to the direction in which the vehicle turns. Switching of the friction force provided to the brake pads from the static friction force to the dynamic friction force is considered one of the causes of this noise.
As described above, when the brake control during turning is carried out on a vehicle in a stopped state and the vehicle starts moving, the driver may feel uncomfortable.